Four modern day teens must figure out a puzzle spanning centuries as they travel through time to save the world from a coterie of evil scientists.Tim is a shy history nerd, Julie is popular and smart, Billy is athletic and confident, and Rose is clever and quirky. But what none of them understand is why Steven Hopkins, a Time Traveler from the 24th century, has chosen them to take on the Emperors of Time. The four teens must travel back to 1916 San Francisco to stop the past from being changed. If they fail, the self-proclaimed Emperors of Time will control the future. Their adventure takes them from a bridge over the Susquehanna River in the days before the Battle of Gettysburg to a settlement of outcasts from the Emperors' futuristic paradise and from the seediest to the swankiest parts of 1916 San Francisco.American History plays as the backdrop for this YA Time-Travel novel.